The present invention relates to a method and apparatus for molding and cooling plastic molded articles such as preforms made of single or multiple materials such as plastic resins. In particular, the invention teaches a rapid injection molding process where the molded articles, such as PET preforms, are ejected from the mold before the cooling step is complete. This is possible as a result of the utilization of a new post-mold cooling process and apparatus where the preforms are cooled internally by convection heat transfer, after being removed from the mold and retained outside the mold area. The present invention also teaches additional external cooling, done through either convection or conduction heat transfer, which may take place at least partially simultaneously with the internal cooling.
Proper cooling of molded articles represents a very critical aspect of the injection molding process because it affects the quality of the article and impacts the overall injection cycle time. This becomes even more critical in applications where semicrystalline resins are used, such as the injection molding of PET preforms. After injection, the PET resin remains in the mold cavity space for cooling for a sufficient period of time to prevent formation of crystalline portions and to allow the preform to solidify before being ejected.
Two things typically happen if a preform is rapidly ejected from a mold in order to reduce the cycle time of the injection process. The first is that the preform is not uniformly cooled. In most instances, the bottom portion opposed to the mold gate is crystallized. The amount of heat accumulated in the walls of the preforms during the injection process will still be high enough to induce post molding crystallinity especially in the gate area of the preform. The gate area is a very critical spot because cooling of the mold in this portion is not effective enough and also because the resin in the mold cavity space is still in contact with the hot stem of the hot runner injection nozzle. If this area of a preform remains crystalline above a certain size and depth, this will weaken the quality of a blown article. The second is that the preform will be too soft and thus can be deformed during the next handling steps. Another critical area of a preform is the neck finish portion which in many instances has a thicker wall and thus retains more heat than the other portions. This neck portion needs aggressive post-mold cooling to prevent it from becoming crystallized. Also aggressive cooling tends to make the neck solid enough to sustain further manipulations.
Many attempts have been made in the past to improve the cooling efficiency of PET injection molding systems, but they have not resulted in a significant improvement in the quality of the molded preforms or a substantial reduction of the cycle time. Reference is made in this regard to the U.S. Pat. No. 4,382,905 to Valyi which discloses an injection molding method where the molded preform is transferred to a first tempering mold for a first cooling step and then to a second tempering mold for a final cooling step. Both tempering molds are similar to the injection mold and have internal means for cooling their walls that make contact with the preform during the cooling process. Valyi '905 does not teach the provision of cooling devices located on the means for transferring the preforms from the molding area or additional cooling devices that circulate a fluid coolant inside the molded parison.
U.S. Pat. No. 4,592,719 to Bellehache discloses an injection molding method for fabricating PET preforms where molded preforms are removed from the injection cores by a first movable device comprising vacuum sucking devices for holding the preforms and also comprising air absorption (convection) cooling of the outer surface of the preform. A second cooling device is used by Bellehache '719 in conjunction with a second movable device to further cool the inside of the preforms also by air absorption. See FIG. 22 herein. Bellehache '719 does not teach cold air blowing inside a preform which has a significantly higher cooling effect with respect to sucking or absorbing ambient air and also does not teach cooling means by conduction heat transfer located in intimate contact with the preforms wall and air blow means directed to the dome portion of the preforms. Bellehache suffers from a number of deficiencies including less cooling efficiency, less uniformity, longer cooling time, high potential for preform deformation.
U.S. Pat. Nos. 5,176,871 and 5,232,715 show a preform cooling method and apparatus. The molded preform is retained by the injection molding core outside the mold area. The mold core is cooled by a coolant that does not make contact with the molded preform. A cooling tube larger than the preform is placed around the preform to blow cooling air around the preform. The principal problem with the apparatus and method shown in these patents is that the preform is retained in the mold core and this significantly increases the cycle time. Also internal cooling is not achieved by direct contact between coolant and the preform.
Further reference is made to U.S. Pat. Nos. 5,114,327, 5,232,641, 5,338,172, and 5,514,309 that teach a preform internal cooling method using a liquid coolant. Preforms ejected from a mold are transferred to a preform carrier having vacuum means to retain the preforms in place without making contact with the preforms' external wall. The preforms carrier however does not have any cooling devices. Cooling cores are further introduced inside the preforms retained by the carrier and a cooling fluid is blown inside the preforms to cool them. The coolant is further removed by the same vacuum means that retain the preforms from the chamber surrounding the preforms. These patents do not teach blowing cold air inside a preform where the air freely leaves the preform after cooling. These patents also do not teach simultaneous cooling of the preforms internally and externally or a preform carrier having cooling means. See FIG. 21 shown herein.
Further reference is made to Japanese Pat. Discl. 7-171888 which teaches a preform cooling apparatus and method. A molded preforms robot carrier is used to transfer the preforms to a cooling station. The robot includes external cooling of the preforms walls by conduction thermal transfer using a water coolant. The cooling station comprises a first movable transfer robot that has a rotary hand portion including vacuum means for holding the preforms and also external cooling of the preforms walls by conduction thermal transfer. The molded preforms are transferred from the robot carrier to the hand portion. The hand portion is moved from position A to position B where it is rotated by 90° in order to transfer the preforms (cooled so far only at the exterior) to a cooling tool. The cooling tool has means to hold the performs, devices to cool the inside of the preforms by blowing air and devices to cool the outside of the preforms by either blowing air or water cooling. The internal cooling which is employed is shown in FIGS. 19 and 20 herein. This patent does not teach a cooling method where internal and external cooling are performed as soon as possible from the moment the preforms are ejected from the mold and into a carrier plate. It also does not teach simultaneous internal and external cooling of the preforms while they are retained by the movable robot carrier. Therefore, this cooling method is not fast enough and does not prevent crystallinity formation outside the mold.
FIGS. 19 and 20 show known methods of internally cooling preforms where a cooling device is located outside the preform and is used to blow cool air inside the preform. Because the air nozzle is located outside the preform, the incoming cold air flow will inevitably interfere and mix at least partially with the outcoming warm flow. This will significantly reduce the cooling efficiency. If the cooling device is on the same axis with the preform, the approach of FIG. 19 is ineffective because there is no air circulation in the preform. If the cooling device is laterally shifted as in FIG. 20, internal air circulation is achieved, but this is still ineffective because one side of the preform is better and faster cooled than the other. The coolant has a quasi-divergent flow profile with a non-symmetrical profile. This profile is very ineffective and it does not allow to concentrate the cooling fluid/gas towards the sprue gate or dome portion.